docNumbers - an array for reuse. An implementation should use the
passed array as a container for the return value,
unless the length of the returned array is different
from docNumbers. In which case an
implementation will create a new array with an
appropriate size.

release

Releases this index reader and potentially frees resources. In contrast
to IndexReader.close() this method
does not necessarily close the index reader, but gives the implementation
the opportunity to do reference counting.